Does blue sealant really need the 24hrs it says on the back before you can refill with oil/water?

Depends upon what sort of blue sealant it is. Is it hylomar blue, or some other make?

Good point, it is Loctite 595 - Blue Silicone RTV Compound.

Don't know about that, never used it. I presume it says to leave it for 24 hrs on the tube?

Yep, I guess I will just have to be patient - if it means a good seal I am happy.

Loctite tend to know how to stick stuff together so I'd go with whatever they say. Bit of a git if you're waiting on it, but at least it'll be sealed.

Water pump gaskets shouldn't need silicon to make them seal ! Hylomar if anything then you don't need to wait . Most silicons start to go off when exposed to atmosphere